1. Technical Field
The present invention relates to an image processing method and an image processing apparatus in which image quality degradation can be prevented by a filter process and an image forming apparatus including the image processing apparatus.
2. Description of Related Art
Between input of an image through an image input device such as a scanner or a digital camera and output of an image through an image output device such as a display or a printer, input image data is subjected to a combination of several image processes. As an image sensor of an optical input device such as a scanner or a digital camera, a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor, a CIS (Contact Image Sensor) or the like is used. Furthermore, in a scanner or a digital camera, light collected through a lens is converted through photoelectric conversion by an image sensor into an electric signal, the resultant electric signal is subjected to A/D conversion for obtaining a sensed image as digital data, an image process is performed on the digital data for converting the data into a format according to an image output device, and the thus obtained image is outputted.
When, for example, a device designated as a multi-function printer or a digital copying machine including, as an image input device, a scanner using a linear line CCD sensor as an image sensor and including, as an image output device, an inkjet or laser printer is exemplified, several image processes are performed before a sensed image of a document read by the scanner is outputted to the printer as described in Japanese Patent No. 3472479. When an image process is performed by using an input sensed image, for example, a visual sensitivity correction process, a filter process and the like are performed in general.
The visual sensitivity correction process is performed for adjusting a digital data value of a sensed image in accordance with the visual characteristic of a person. The conversion characteristic of an image obtained through an image sensor and an A/D conversion process is generally a linear characteristic, and a digital data value is increased as the quantity of light entering an image sensor is increased. In contrast, the visual characteristic of a person is known to be logarithmic against the quantity of incident light. Therefore, the visual sensitivity correction process is performed in order to ease a subsequent image process by converting a digital data value obtained after the A/D conversion into a digital data value accordant with the visual characteristic of a person.
The filter process is performed for various purposes, and for example, it is performed for improving a degraded spatial frequency characteristic of a sensed image derived from the MTF (Modulation Transfer Function) characteristic of an optical lens used in an optical input device. In general, a digital data obtained after subjecting a sensed image to the visual sensitivity correction is used as an input image in the filter process. For example, Japanese Patent Application Laid-Open No. 61-157169 discloses a spatial filter process in which a result of an edge enhancement process and a result of a smoothing process obtained from an image are mixed on the basis of a result of edge discrimination. Alternatively, Japanese Patent Application Laid-Open No. 10-271340 discloses an image filter for mixing a result of a filter operation of an image with the original image in a mixing ratio calculated based on contrast. Furthermore, Japanese Patent Application Laid-Open No. 7-288768 discloses an image filter for switching the content of a smoothing process by using a difference in the pixel value between a current pixel and a surrounding pixel.